Warren Buffett's Berkshire Hathaway (BRK.A -0.57%) (BRK.B -0.71%) stock portfolio is one of the world's most closely followed collections of stocks. The reason is simple: Buffett has led Berkshire to market-crushing gains over a long period of time. A $1,000 investment in Berkshire Hathaway at its IPO would be worth $2.4 million today.Berkshire Hathaway's success is due to several factors, but the part that most investors pay attention to is Buffett's stock-buying decisions. Of interest lately has been the ...